By "the joiner piece" do you mean the cable between the tips? If it looks white in a fixed circular, it probably won't get better on its own. You can dip it, not the needles or the place they're attached, in hot water and let it hang and that will help but it will recoil no matter what you do. If you really do end up doing a queen size blanket all in one piece, you might want to consider looking at other needles. I think I'd do 3 sections and seam them to avoid a seam right down the middle.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper